【作者】 李强；

【导师】 张人禾；

【作者基本信息】 中国气象科学研究院 ， 气象学， 2011， 博士

【摘要】 冬春季节，青藏高原对西风带有明显的分支绕流作用，并由此对东亚环流和下游天气产生了重要的影响。本文首先使用NCEP/NCAR再分析资料分析了西风绕流的季节变化特征；然后进一步研究了冬季南支绕流的年际变化，并根据中国地面气候资料日值数据集分析了冬季南支绕流年际变化对中国降水的影响；最后对南支绕流的强弱变化机制进行了分析研究。结果表明，西风绕流的季节变化可以用青藏高原西侧辐散线（v=0等值线）的南北移动来表示。由冬到夏的季节转换过程中，辐散线（西风绕流）在17候和22候有二次明显的北跳，然后于28候消失；在由夏向冬的季节转换过程中，辐散线（西风绕流）于59候重新出现。低层西风绕流的这种变化与高层西风急流的季节演变是类似的，都具有非常明显的突变特征。由冬季向夏季转换期间，低层西风绕流的消失领先于高层西风急流轴的北跳；但由夏季向冬季转换的过程中，低层绕流的出现落后于高层西风急流的南落。西风绕流与高原的热力状况有密切的关系，在季节转换过程中，青藏高原由冷源转为热源（热源转为冷源）的时间要早于绕流消失（出现）的时间。冬季青藏高原绕流的变化主要是高原上游西风的强弱变化引起的，用高原西侧西北风定义的南支绕流强度指数对与南支绕流有关的大范围环流特征有很好的表示，能完整反应绕流的意义和信息。南支绕流强度指数存在明显的年际变化和年代际变化，变化的主要周期是2~4年，同时还有8~10年周期。南支绕流的年际变化对我国冬季降水有重要的影响，当南支绕流加强时，我国长江以南地区降水会增多，而在南支绕流减弱的时候，东部地区降水减少。南支绕流对中国冬季降水的影响是通过与之相关联的环流异常造成的，即当南支绕流异常加强时，赤道附近和我国长江以南地区会出现反Hadley环流异常，在赤道附近下沉运动增强，我国东部地区上升运动加强，在对流层低层的南风会向北输送大量的水汽，水汽在长江以南地区辐合抬升，造成降水异常。影响南支绕流强弱的主要因子包括AO、青藏高原热力和积雪状况以及东亚冬季风等。AO是对青藏高原以西区域南支绕流上游西风强弱具有最重要意义的外部信号。当AO正位相时，在700hPa从北大西洋到阿拉伯海一带会出现西风异常和东风异常交替的西北—东南向波列，这一波列的出现加强了西亚地区的西风，为强南支绕流的出现提供了前提。而上游的强南支西风盛行到下游所对应的条件是亚洲中低纬气压降低、东印度洋和南海一带副热带高压加强。在AO正位相的波列中，热带洋面上出现的东风异常有利于其上空反气旋的形成。冬季，青藏高原冷源异常偏弱时，上游西风加强，引起高原西部和东南部降水增多，潜热释放增加，导致高原冷源减弱，从而进一步加强上游的西风，形成了一个正反馈的过程。同时，青藏高原冷源的减弱还会引起西亚和青藏高原附近地区气压的下降，有利于北低南高异常气压场的出现。当青藏高原积雪偏多时，会引起东印度洋上空的异常下沉运动，这也会对副热带高压起到加强作用。东亚冬季风与南支绕流存在一种反相的关系，强的冬季风实际上是经向环流加强，纬向环流减弱的一种环流形式，而南支绕流的加强则是纬向环流增强的形式，故二者存在一定的反向关系，当东亚冬季风偏弱时，亚洲大陆气压也会下降，对南支绕流的盛行亦有贡献。因此，本文的结果表明，AO、青藏高原热力、积雪状况以及东亚冬季风的共同作用是引起南支绕流变化的原因。更多还原

【Abstract】 In the winter and spring, obstructed by the Tibetan Plateau, the low levelwesterly is split into two branches, which flow around the plateau from northside and south side respectively, thus exerts a significant influence on EastAsia circulation and its downstream weather. Utilizing the NCEP/NCARreanalysis data, we first analyzed the seasonal changes of the circumferentialwesterlies. Then based on the daily surface climatic data of China, the annualvariablity of the southern branch of western flow (SBWF) and its effect on thewinter precipitation in southern China are discussed. Finally, we try to find outthe mechanism causing the strong and weak SBWF.We find out the shift of the Divergence Line (DL, v=0contour) in the westof the Tibetan Plateau can indicate the seasonal change of circumferentialwesterlies. During the season transition in winter to summer, the DL has twosignificant northward jumps at17th and22nd pentad and disappears at28thpentad, then the DL rebuilds at59th pentad. The northward jump of DL in lowtroposphere is rapid, which is similar to the change of westerly jet in up level.During the transition in winter to summer, the revolution of DL precedes thenorthward jump of westerly jet, but lags behind the southward jump of westerlyjet during the transition in summer to winter. The thermal state of TibetanPlateau plays a key role during the revolution of circumferential westerlies.The change of circumferential flow around the Tibetan Plateau is mainlycaused by the strength variability of the westerly in the upstream. With thenorthwest winds in west of Tibetan Plateau, we define an intensity index ofSBWF, which can reflect the large-scale circulation features well. The indexshows an obvious inter-annual and inter-decadal variability, the period is2to4years and8to10years. Its inter-annual variability plays important impact on precipitation in winter over south China. Strong SBWF increases winterprecipitation over the region in the south of Yangtze River and weak SBWFdecreased the precipitation in the east of China. These effects are associatedto the anomaly of the related circulation. When SBWF strengthens anomaly,the anomalous Anti-Hadley circulation between the equator and the south ofYangtze River develops. So the sink motion near the equator and the risingmotion over the East China are enchanced. Then more water vapor istransported by the southerlies in the low level. Under the favorite convergencecondition, more precipitation occurred.The main factors that affect the strength of SBWF include AO, thermal andsnow cover condition of Tibetan Plateau and East Asian Winter Monsoon. AndAO is the key signal affecting the strength of westerly in the upstream ofTibetan Plateau. With the positive AO, a northwest-southeast direction wavetrain appears from north Atlantic to Arab Sea, which strengthens the westerlyin West Asia and is favor of the enhancing of SBWF.However, the strong prevail SBWF condition causing strong westerlydownstream is the weakening of pressure over middle and low latitude of Asiaand the strengthening of subtropical high pressure over the East Indian Oceanand South China Sea. The easterly over the tropical ocean associated withpositive AO benefits the formation of the above anticyclone. In winter, theweakening of cold source over Tibetan Plateau can cause the strengthening ofwesterly in the upstream. This leads to more precipitation and latent heatrelease increasing, which can weaken the cold source of Tibetan Plateau more.Thus a positive feedback is formed. Meanwhile, the weakening of cold sourceof plateau can descend the pressure over west Asia and plateau, which isconducive to the appearance of such pressure field with low in north and highin south. More snow in plateau can cause downward motion over East IndianOcean, which can strengthen the subtropical high. SBWF shows a negativerelationship with East Asian Winter Monsoon. The meridional circulationstrengthens during strong winter monsoon and the zonal circulation enhancesin company with the strong SBWF, and vice versa. Therefore, we can concludethat AO, thermal and snow condition of the Tibetan Plateau and East Asianwinter monsoon cause the change of SBWF.更多还原